Within the scope of measures to downsize internal combustion engines, the displacement of the internal combustion engine is continuously reduced and the specific power output per liter is increased accordingly. In order to make this performance increase possible, an increased fuel quantity having an increased air-mass throughput is necessary.
The air-mass throughput may be increased by increasing the pressure in the intake manifold of the internal combustion engine with the aid of a supercharging device which is in particular exhaust gas-driven. A performance increase of supercharging devices of this type may be generally achieved by increasing the diameter of their exhaust gas turbines and their compressors. Also, this supercharging device must be capable of providing a constantly high charging pressure level across a high rotational speed range. In particular, in the case of lower rotational speeds, a supercharging device having an increased diameter of the turbine results in a deteriorated acceleration behavior.
In order to compensate for this disadvantage, the supercharging device may include an electromechanical supercharger drive for the purpose of supporting the exhaust gas-operated supercharging device with the aid of the electric motor or by designing the supercharging device completely in the form of an electrically operated supercharging device.
German Patent Application No. DE 10 2004 038 156 A1, for example, describes exhaust gas-operated supercharging devices operated, in general, based on an upstream supercharging pressure control unit which predefines a setpoint rotational speed of the compressor as the reference variable to provide a supercharging pressure required by the internal combustion engine. In the case of conventional exhaust gas-operated supercharging devices, this reference variable is set by manipulating a so-called supercharger actuator, such as a waste-gate valve or a VTG actuator. In the case of supercharging devices being supported by an electric motor or in the case of a supercharging device operated purely by an electric motor, this rotational speed is set with the aid of a rotational speed control of the supercharger drive. This rotational speed control predefines as the control variable an actuating torque in the form of a setpoint motor current which is used to actuate the supercharger drive directly in conjunction with a suitable pilot control, for example, or with the aid of a suitable current controller.
In the case of dynamic load curves, in particular, the supercharger drive is to be operated with the aid of the rotational speed device in such a way that the acceleration of the compressor of the supercharging device is made possible at a preferably steep rotational speed gradient.